1. Field
Embodiments and implementations of the present invention relate generally to image-transferring arrays such as fiber optic faceplates and other optical fiber imaging devices and more particularly to the inclusion of integral contrast enhancement in such devices.
2. Brief Description of an Illustrative Environment and Related Art
Fiber optic face plates (FOFPs) are exemplary of image transfer devices generally comprising coherent lattice arrays of step index waveguides that behave as image plane transfer devices; that is, they conduct an image from an input surface to an opposite output surface. Other known image transfer devices include elongated image conduits, microconduits, fiber optic tapers, fiber optic image inverters, flexible image scopes, light guides and individual optical fibers by way of non-limiting example.
A fragmentary cross-section of a fiber optic face plate 10 of a general type known to those of ordinary skill in the art is shown in FIG. A. The face plate 10 comprises a well-ordered array of two-phase optical waveguides (e.g., optical fibers) each of which waveguides includes a core 14 characterized by a first refractive index n1 surrounded by a contiguous second phase cladding 16 characterized by a second refractive index n2 that is lower than the first refractive index n1. Glasses commonly used in the fabrication of fiber optical faceplates include high-refractive-index glasses for the core material and low-refractive-index glasses for the cladding. The core 14 extends along a core axis AC. Immediately adjacent to the cladding 16, on either side of a core 14, are neighboring cores 14. In a typical fiber optic faceplate used in imaging applications, the waveguides are packed in a well-ordered lattice. Illustrative, non-limiting dimensions include a center-to-center spacing of 6 microns and a core diameter of 5 microns.
The mode of light transmission in imaging devices like the faceplate 10 in FIG. A is considered relative to a single core 14 as follows; wherein the core axis AC is treated as normal to the input surface through which incident light enters the faceplate 10. An incident light ray R1 enters the core 14 at an incident angle θi relative to the core axis AC. In accordance with Snell's Law, if the sum (90°−θi) is greater than the critical angle θc for total internal reflection, the incident ray R1 is not refracted into the cladding 16 but is instead reflected down the core 14, as shown. Incident rays R1′ for which the sum (90°−θi) is less than θc are at least partially refracted and pass, at least partially, into the cladding 16, where they are free to enter adjacent cores 14. This phenomenon is the most commonly discussed mode of cross-talk between constituent fiber elements in fiber optic faceplates.
A common measure of the acceptance angle of a waveguide, within which total internal reflection occurs, is the Numerical Aperture (N.A.), defined as N.A.=sin(90−θc)=(n12−n22)1/2. Accordingly, for example, in the case in which the N.A. of each waveguide in the FOFP is 1.0, θc=0°, and light up to 90° off normal incidence is totally internally reflected. For an N.A.=0.5, θc is 60°, and incident light of up to 30° off normal incidence is totally internally reflected.
Also shown in FIG. A is an incident ray R1″ entering the cladding 16 instead of the core 14 at incident angle θi (i.e., parallel to R1). In this case, instead of being totally internally reflected, the ray R1″ is partially refracted (shown as a dotted line) into the core 14. The other portion of the incident ray R1″ (shown as a solid line) is reflected back into the cladding 16 which, because the cladding 16 is a common phase, results in dissipation over a wide area. Portions of reflected rays also enter adjacent cores 14 at each subsequent reflection, leading to cross-talk. This type of cross-talk is not commonly considered a source of image quality degradation in fiber optical faceplates, but it is, in fact, quite substantial.
Light-absorbing materials (e.g., glasses) are often incorporated into fiber optic faceplates to suppress cross-talk. These absorbing materials, alternatively referred to as Extra-Mural Absorption (EMA) materials, media, glasses, fibers, filaments and rods, as indicated by context, are typically incorporated in accordance with one or more of three general methods. According to a first approach, absorptive coatings—or even sleeves or tubes—are applied to the outside of each waveguide individually yielding what is referred to as a “circumferential EMA.” A second approach indicates the substitution of selected waveguides within a waveguide bundle with absorbing fibers wherein the substitute fibers are referred to as substitutional EMA fibers. According to a third common approach, absorbing fibers are inserted into the interstitial packing vacancies in the array (such fibers are known as “interstitial EMA fibers”).
Although circumferential, interstitial and substitutional EMA media have met with varying degrees of success in suppressing cross talk due to the refraction and propagation of unwanted stray light, the need for black glass tubing and/or individual EMA fibers, in various configurations, invariably adds to the complexity and expense of fabrication and, furthermore, can introduce aberrations into transferred images. Moreover, the introduction of different glass compositions in an array increases the potential for adverse interactions between incompatible glasses.
Attempts have been made to reduce cross-talk, not by incorporating EMA materials along the lengths of arrays, but by preventing light from entering the cladding material at the face of an array. One method of preventing light from entering cladding material at the face includes using a cladding material containing reducible ions and then, after completion of an array product, including grinding and polishing, for example, exposing the array to a reducing atmosphere to blacken the cladding at a face of the array. As observed previously in this background section, however, light entering non-blackened cladding accounts for only a portion of cross-talk-yielding stray light; a portion of light entering the cores of a smooth-faced array also produces cross-talk. In recognition of this observation, Cook, in U.S. Pat. Nos. 5,259,057 and 5,351,332, discloses a waveguide array with an intagliated image-input surface. As explained by Cook, “the surface of the FOFP is intagliated with a cavity for each waveguide formed in the face thereof. Each cavity has side walls formed of exposed cladding. The end of the core is recessed relative to the cladding. The exposed cladding, including the side walls of the cavity, is also reduced to produce the darkened surface layer, resulting in an effective numerical aperture which eliminates cross-talk from core incident light.” It will be appreciated that the intagliation and blackening by Cook reduces cross-talk caused by core-incident light by absorbing, in the blackened-cladding cavity wall, light rays that, in the absence of intagliation and subsequent blackening, would enter the core at angles greater than the critical angle as measured with respect to the core axis.
Although the intagliation and blackening in Cook constitute an apparent improvement over blackening the cladding material only in the plane of the array face, the Cook method sacrifices the benefits of including EMA material throughout the length of an image-transferring array.
Accordingly, there exists a need for a method of reducing cross-talk in optical fiber image conduits that obviates the use of traditional EMA media (e.g., black glass tubing and interstitial fibers) while, in various implementations, providing light-absorbing material along the full length of an image conduit between image input and image output faces.